The present invention generally relates to nanoporous structures, and particularly to forming porous nanoscale metal structures by reactive ion etching.
Nanoscale (i.e., less than 100 μm) structures (“Nanostructures”) are increasingly having applications in numerous fields, such as metal-ion batteries, energy storage and chemical sensors. One such application is the use of palladium nanowires as hydrogen (H2) sensors. Palladium (Pd) is capable of absorbing hydrogen to form a palladium hydride (PdHx, 0.0<x<˜0.67). Because the resistance of the palladium increases as hydrogen absorption increase, a Pd nanowire may be used to sense hydrogen by measuring the conductance across the Pd nanowire. A decrease in conductance may be correlated with an increase of hydrogen concentration in the region around the nanowire.
Nanostructures such as nanowires are particularly useful as chemical sensors due to their high surface-to-volume ratio. For example, because a palladium structure with a high surface-to-volume ratio can more easily absorb hydrogen, it may be more sensitive to environmental changes relative to non-nanoscale structures with lower surface-to-volume ratio. Because there is a correlation between surface-to-volume ratio and sensor performance, it follows that it may be advantageous to further increase the surface-to-volume ratio of palladium nanowires to further increase the sensitivity.
One manner of increasing the surface-to-volume ratio of a nanostructure is to increase it's porosity by forming a plurality of voids in the nanostructure. However, typical processes for forming porous nanostructures require bottom-up manufacturing approaches such as solution phase reactions or dispersion on a surface. Such processes may be difficult to control, scale up, or incorporate into typical fabrication flows. Therefore, a process of forming a porous nanostructure, for example a porous palladium nanowire, using typical semiconductor fabrication processes may be desirable.